Fig. 6. Electron energy dependence of the electron-near-field coupling strength.

BEM calculation of the laser-induced electron-near-field coupling strength ${∣\beta ∣}^2$ as a function of the along-the-beam wave vector $q=\omega /v$ and impact parameter d relative to the tip apex. Data are obtained for a fixed photon energy of $ħ\omega$=1.73 eV matching the tip resonance energy and for varying electron energies E₀ (see top axis). Other excitation parameters are identical to those in Figs. 4c and 5d. The inset shows a profile along the white dashed line for d = 3 nm, representing the local spatial Fourier composition of the optical field along the electron trajectory. For a given E₀, the electron can only couple to the Fourier field component with a spatial phase advance q, affecting both the amplitude and the spatial distribution of ${∣\beta ∣}^2$